The present invention relates to an aperture correction circuit which corrects the aperture aberration of the video signal of a television receiver in order to reproduce a sharp image.
An aperture correction circuit in the prior art has been as shown in FIG. 4.
FIG. 4 is a block diagram showing the circuit arrangement of the prior-art example, while FIGS. 5A-5F are waveform diagrams of the signals of various portions within the circuit shown in FIG. 4.
Referring to FIG. 4, when a signal S.sub.0 is received at a video signal input terminal 1, a signal S.sub.1 delayed through a delay circuit 2 and a signal S.sub.2 further delayed through a delay circuit 3 are obtained. First, the signals S.sub.0 and S.sub.2 are added by an adder 43 to produce a signal S.sub.A Since this signal S.sub.A becomes a signal as indicated by S.sub.A in FIG. 5D, an aperture correcting shoot signal S.sub.B (shown in FIG. 5E) can be obtained in such a way that the signal S.sub.1 being the output of the delay circuit 2 is subtracted from the signal S.sub.A by a subtracter 44.
In this case, assuming that the signal amplitudes of the signals S.sub.0, S.sub.1 and S.sub.2 are equal, also the maximum amplitude of the output S.sub.A of the adder 43 needs to be equal to them, and this is as illustrated in FIG. 5D. Further, the output S.sub.B of the subtracter 44 and the output S.sub.1 of the delay circuit 2 are added by an adder 45, whereby a video output S.sub.C after the correction of an aperture aberration is obtained at an output terminal 46. The waveforms of the signals in FIG. 4 become as shown in FIGS. 5A-5F.
As seen from the waveform S.sub.C in FIG. 5F, the ratio between the amplitudes of a preshoot signal (a) and an overshoot signal (b) has generally been 1:1 with the prior-art circuit arrangement. Alternatively, the amplitude ratio between the preshoot signal and the overshoot signal can be set as being somewhat unbalanced. Since, however, the balance between the amplitude ratio of the shoot signals is fixed to the last, the prior-art system has difficulty in attaining image qualities in correspondence with various video input sources or input conditions.
An aperture correction circuit capable of changing the amplitudes of the shoot signals is disclosed in the official gazette of Japanese Patent Application Laid-open No. 56-68073.
Meanwhile, conventional aperture correction circuits have generally employed a method in which a shoot signal prepared from a video signal is directly added to the original signal without being reprocessed. Therefore, blooming ascribable to the shoot signal appears at the bright part of a screen. As a circuit in which the relief of the blooming is considered, there has been employed an arrangement as shown in the circuit diagram of TH-26B1 or TH-29B1 on p. 145 in "Service Book for All Manufacturers" which is an extra issue of "Terebi Gijutsu (Television Engineering)" dated Sept. 20, 1987.
The prior-art circuit arrangement is shown in FIG. 6, while the waveforms of signals at several parts in FIG. 6 are shown in FIGS. 7A-7D. Referring to FIG. 6, a signal S.sub.0 received at a video signal input terminal 1 is applied to a delayed video signal terminal 51 for a video chroma IC 60 (in FIG. 6, only a video circuit is shown in blocks) through a delay line 2. The signal S.sub.0 is further applied to a video signal input terminal 50 from the input side of the delay line 2. The applied signals are subjected to reflection-type delay-line aperture correction processing by an image quality adjuster 56, thereby to compose a shoot signal. The amount of the shoot signal to be superposed on the video signal is varied according to a control signal at an image quality adjusting input terminal 52, whereby the image quality of the video signal is adjusted. The resulting output SA is delivered to a luminance signal output terminal 68 for the IC 60 via a contrast adjuster 57, a pedestal clamp circuit 58 and a luminance signal amplifier 59. The output of the terminal 68 is amplified by an emitter follower of Darlington connection which is configured of an N-P-N transistor 71, a P-N-P transistor 72, and resistors 70, 73. The amplified output signal is delivered from a luminance signal output terminal 75 to a CRT drive circuit at a succeeding stage. On the other hand, the amplified output signal is passed via a resistor 69 and is further amplified by an emitter follower circuit which is configured of a P-N-P transistor 67 and a resistor 66. The resulting output is divided by resistors 61, 63 and a variable resistor 62, and the divided voltage is applied as the control signal to the image quality adjusting input terminal 52. In this way, the image quality, namely, the amount of the shoot signal to be added to the original signal of the video signal is controlled according to the output amplitude of the output luminance signal, thereby to make the corrections of decreasing the amount of the shoot signal at a bright part on a picture frame so as to relieve blooming and increasing the amount of the shoot signal at a dark part.
The signal waveforms at several parts in the arrangement become as illustrated in FIGS. 7A-7D. The original signal S.sub.0 of the video signal (in FIG. 7A) and the shoot signal S.sub.S composed from the signal S.sub.0 (in FIG. 7B) are added by the image quality adjuster 56 in FIG. 6, to become as indicated by dotted lines at S.sub.A (in FIG. 7C). The signal waveform at the luminance signal output terminal 68 in FIG. 6, and the waveform S.sub.B (in FIG. 7D) at the emitter follower output 75 in FIG. 6 become waveforms of dotted lines accordingly. Since, however, the waveform S.sub.B (in FIG. 7D) is used for the control of the image quality adjustment in the circuit arrangement, actually the signal output S.sub.B (in FIG. 7D) at the emitter-follower output terminal 75 becomes as indicated by a solid line due to a delay time involved in the course from the output of the image quality adjuster 56 to the image quality adjusting input terminal 52, and the aperture aberration is not properly corrected at the leading edge of an aperture in principle. Another problem is that, since the amount of the shoot signal is larger at a part of lower luminance, the S/N (signal-to-noise) ratio of the video signal degrades.
With the prior-art aperture correction circuit, the ratio between the amount of the preshoot signal and that of the overshoot signal is fixed, and hence, it has been impossible to adjust the image quality in consideration of even the shoot balance corresponding to the applied video signal.
The timing of the composition of the video signal and the aperture correction signal is not sufficiently considered, either, so that the image quality adjustment at the leading edge of the aperture has been unsatisfactory.